Laser-induced fluorescence and absorption spectroscopy are used to study the mixing of populations of excited He atoms due to electron collisions in the negative glow of a dc He discharge. These measurements yield the density (${\mathit{n}}_{\mathit{e}}^{\mathit{c}}$≃5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}3}$) and temperature (0.1 eV≤${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{e}}^{\mathit{c}}$≤0.2 eV) of the low-energy (or ‘‘cold’’) electrons in the negative glow. The cold electrons are trapped in a potential-energy well. In a complementary investigation, Monte Carlo simulations are used to determine the density (${\mathit{n}}_{\mathit{e}}^{\mathit{h}}$≃${10}^9$ ${\mathrm{cm}}^{\mathrm{-}3}$) and temperature (${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{e}}^{\mathit{h}}$≃3 eV) of the high-energy (or ‘‘hot’’) electrons in the negative glow. Results from the experiments and Monte Carlo simulations are combined to study the power balance of the cold trapped electrons. The cold-electron temperature is established by a balance between cooling from recoil during elastic collisions with neutral atoms, and heating due to Coulomb collisions with hot electrons. The linear variation of hot-electron density with discharge current density causes a linear variation in cold-electron temperature with current density. Hot electrons also excite metastable atoms to higher radiating levels to produce most of the light from the negative glow.

We investigate spatially homogeneous stationary solutions of the Boltzmann equation describing the electron component of a gas plasma in a homogeneous electric field. We consider both elastic and weakly inelastic collisions of electrons with the neutral-particle plasma component as well as the Coulomb interaction between charged particles, described by Landau’s collision integral. The Boltzmann equation for the electron swarm is linearized around a Maxwellian with some unknown temperature. For some important cases the problem reduces to a single-parameter system of two integrodifferential equations. Its solution allows us to treat plasmas under a variety of conditions in a rather simple way. We solve this system with the help of the spline-collocation method, and find the first two corrections to the Maxwellian corresponding to symmetrical and flow components of the velocity distribution function. The equation for the electron temperature then comes from the energy balance condition and from the requirement that the solution of the kinetic equation be unique. The electron temperature and energy absorption are described by curves with hysteresis as functions of the electric-field intensity. The conductivity versus electron temperature has a single maximum. Its rising slope corresponds to Spitzer’s well-known formula, when the electron temperature is not very high and electron–neutral-particle collisions are unimportant. All numerical results are obtained under the assumption that the cross section for electron–neutral-particle collisions is independent of electron energy but generalizations are possible, since the analytical solution of the problem is far advanced.

A theoretical study of low-pressure helium rf glow-discharge plasmas is presented. The numerical scheme provides a fully kinetic description of the electrons and singly charged He ions in one spatial dimension and predicts the electric fields present in a mutually consistent fashion. The results have implications for a variety of discharge geometries and gases. Realistic, detailed cross sections for He are used. Both the numerical algorithm and physical processes are discussed. Unlike previous theoretical studies, we find a low average electron energy (<1 eV) in the central bulk region and a weak bulk field (≊2 V/cm peak amplitude) that is out of phase with the strong sheath fields. The discussion emphasizes quantities that can be readily measured in order to facilitate comparison with experiment. Metastable-atom production rates are used as source terms in a diffusion-reaction model to predict metastable-atom densities. Multistep ionization due to metastable-atom–metastable-atom collisions is shown to be important for some of the conditions studied. The pressure dependence of the ion distribution at the electrode is studied. Some common approximations used in modeling discharges are examined for both suitability and possible improvement.

In this work we use the nonlinear properties of the sheath capacitance in a low-density plasma, whose purpose is to produce parametric amplification of a rf signal in the high-frequency band. The experiment has been carried out on the Linear Mirror Device LISA of the Universidade Federal Fluminense, where a helium plasma was produced using a radio-frequency source with a power that can be varied from 10 to 100 W. The experimental results show that it is practical to construct a parametric amplifier with high selective gain, which is very useful for the amplification of weak signals, where the gain factor and the relation between signal and noise are important. We find good agreement between the theoretical model proposed for the sheath capacitance and the experimental results.

A theory is presented of the nonequilibrium space-time development of a stream of electrons moving through a background medium under the influence of a strong electric field. Nonequilibrium processes are treated by supplementing the usual continuity moment equation with equations describing the velocity and energy moments of the electron distribution. It is shown that the theory is able to reproduce many of the important nonequilibrium effects occurring as an electron stream approaches an absorbing anode, although comparison with detailed solutions of the Boltzmann equation does reveal certain limitations apparently inherent in theories based on moment equations.

We derive the kinetic equation governing the evolution of the action density of a wave propagating in a weakly nonuniform, nonstationary medium characterized by fluctuations that are small in magnitude (but need not be slowly varying). The method of derivation is based on the Weyl representation of a general linear wave equation, which leads to a natural definition of the wave-action density and an analysis rooted in the ray phase space. The result is thus a classical wave derivation of the radiation transport equation for a general scalar wave equation. We illustrate our derivation with three example wave equations governing propagation in fluctuating media.

An efficient algorithm for calculating the electronic matrix element ${\mathit{T}}_{\mathit{D}\mathit{A}}$ for electron-transfer reactions between a donor D and acceptor A in biological systems is presented. The tight-binding Hamiltonian and the Green’s-function formalism are used to obtain ${\mathit{T}}_{\mathit{D}\mathit{A}}$ via the renormalized-perturbation expansion. This procedure permits taking into account the detailed structure of the array provided by the specific protein that assists the electron transfer. As an application, results are presented for the calculation of some of these matrix elements in metal-labeled myoglobin, and comparison with experimental data is made. By analyzing the self-energies at each site of the array, these results can be understood as a small-denominator effect.

The vibrational modes involving the hydrogen-bond fluctuations in B conformation of poly(dA)-poly(dT) DNA homopolymer dissolved in aqueous solution have been theoretically studied within the recently developed effective-field model, including longitudinal as well as radial fields, and a properly frequency-dependent dielectric constant for the surrounding water. We have analyzed the vibrational eigenvectors to characterize two types of motions involving hydrogen bonds, namely, propeller twist and hydrogen-bond breathing. We ran our analysis for five different sets of partial atomic charges, and found that the major mode frequencies are essentially independent of the choice of partial charges. Since hydrogen-bond strength changes with temperature, we simulated the effect of temperature on the hydrogen bonds by varying the hydrogen-bond force constants. Our calculations indicate that a mode near 42 ${\mathrm{cm}}^{\mathrm{-}1}$ with strong propeller twist motion is to be expected. Around 63 ${\mathrm{cm}}^{\mathrm{-}1}$, another mode with strong hydrogen-bond breathing character should be found. We discuss the expected variations of these resonances with temperature as a guide to spectral identification.

The statistical distribution of lifetimes in a simple quantum model with absorption is numerically investigated. It displays a number of characteristic features that can be related to the classical diffusive mechanism of absorption, described by a Fokker-Planck equation.

It is shown that the lattice Boltzmann equation arising from the basic lattice-gas scheme for Navier-Stokes equations can be extended in such a way as to include the effects of a two-dimensional magnetic field. This offers the possibility of developing new computational schemes for the efficient simulation of incompressible magnetohydrodynamic flows.

In a recent article by Ou-Yang Zhong-can [Phys. Rev. A 41, 4517 (1990)], rings with generating circles of radii in the ratio 1 √2 have been found as stable equilibrium shapes of vesicle membranes, for some negative values of their spontaneous curvature. Here we report experimental observations of this topology in partially polymerized phospholipid membranes. Evaluating the radii of the generating circles of several toroidal vesicles we found agreement with the predicted value.

We report sensitive gap dependence of morphology in aggregates obtained from electrochemical deposition of zinc in two-dimensional geometry in low concentration and low applied potential limits. The gap effect is similar to the results of noise-reduced or anisotropic diffusion-limited-aggregation simulations. The branching length and angle of the aggregates obtained from digitized images are found to follow Poisson-like and Gaussian distributions, respectively. A diffusion-limited-aggregation branching model captures the experimental results and provides new insights to electrodeposition processes.

A necessary and sufficient condition for the existence of negative-energy modes in a plasma described by the Vlasov-Maxwell system is derived from an expression of Morrison and Pfirsch [Phys. Rev. A 40, 3898 (1989)] for the second variation of the free energy of a steady-state solution.

A theory of photon-plasmon-photon three-wave interaction is presented. An electromagnetic wave launched into the plasma turbulence interacts through the wave-particle interaction resulting in the emission of an electromagnetic wave with frequency up-shifted. The theory is based on the Dirac relativistic wave equation, where the interaction of plasma particles with electromagnetic waves is described. It is shown that the frequency up-shifting of the electromagnetic wave can take place in a current carrying plasma.

Recently Frycz and Infeld performed a numerical calculation that yielded a stage-by-stage picture of a spontaneous transition from flat to an array of cylindrical solitons [Phys. Rev. Lett. 63, 384 (1989)]. The model used was the Zakharov-Kuznetsov equation for solitons in plasmas permeated by very strong magnetic fields. Subsequently, similar calculations were performed for another equation, that of Hasegawa and Mima [Su, Horton, Morrison, and Pavlenko (unpublished)]. In this Brief Report we look at some calculations that endeavor to explain both the collapse of a flat soliton, leading to cylindrical entities, and also the collapse of cylindrical solitons when instabilities along the axes are allowed. Saturation is found in some cases but not in others.

The interaction of highly charged ions of a laser-produced plasma with a background gas medium is studied through space-resolved x-ray spectroscopy in the expansion region of up to several millimeters. Two regions of enhanced x-ray line emission from H- and He-like magnesium ions are observed in the plasma produced by a 3-GW, 5-ns Nd:glass laser, expanding in either helium or nitrogen gas in the pressure range 0.1–10 Torr. The role of three-body recombination and charge-exchange processes in populating the excited states of ions during the interaction is discussed.

The apparent fundamental inconsistency in the behavior of double-time Green’s functions and the correlations related to them by the spectral theorem claimed by Zhang and Qin [Phys. Rev. A 36, 915 (1987)] is shown to arise from the use of an incorrect version of the spectral theorem. The ‘‘method of undetermined constants’’ constructed by Zhang and Qin to remove this apparent inconsistency is not an intrinsic characteristic of the boson Green’s-function method. Rather, it is unnecessary and leads to incorrect results in both systems considered by Zhang and Qin, i.e., the mean-field transverse Ising model and the symmetrically decoupled zero-field Heisenberg model. Use of the correct spectral theorem is shown to remove the inconsistency. Correct treatments based on the correct spectral theorem of the models considered by Zhang and Qin are also presented.

In this Reply to the preceding Comment by Brown [Phys. Rev. A 43, XXXX (1991)], the following two points are discussed. (1) The undetermined-constant method solves problems by using only boson Green’s functions, but the method given by Brown in the Comment must use both boson and fermion Green’s functions. (2) When the undetermined-constant method is applied to the example given by Ramos and Gomes [Nuovo Cimento 3A, 441 (1971)], exactly the same results can be obtained.

The scaling behavior of the interface in a parallel growth model with long-range spatially correlated noise is investigated in two dimensions. Excellent agreement with the predictions of Medina et al. [Phys. Rev. A 39, 3053 (1989)] is found for both the ballistic deposition and restricted solid-on-solid versions of our model. In particular, a transition from uncorrelated exponents to continuously varying exponents is observed, in disagreement with the recent prediction of Zhang [Phys. Rev. B 42, 4897 (1990)] and the numerical results of Margolina and Warriner [J. Stat. Phys. 60, 809 (1990)].

We study a stochastic model of an interface moving through a random medium. This model differs from the standard Kardar-Parisi-Zhang equation by the fact that the fluctuations are quenched random variables. We find an intermediate scaling regime with roughness exponent approximately equal to 0.75; this compares favorably with recent experiments [M. A. Rubio, C. A. Edwards, A. Dougherty, and J. P. Goilub, Phys. Rev. Lett. 63, 1685 (1989)] on multiphase flow through a bead pack.